Estrogen promotes growth of human thyroid tumor cells by different molecular mechanisms

Thyroid tumors are about 3 times more frequent in females than in males. Ep idemiological studies suggest that the use of estrogens may contribute to t he pathogenesis of thyroid tumors. In a very recent study a direct growth s timulatory effect of 17 beta -estradiol was demonstrated in FRTL-5 rat thyr oid cells. In this work the presence of estrogen receptors alpha and beta i n thyroid cells derived from human goiter nodules and in human thyroid carc inoma cell line HTC-TSHr was demonstrated. There was no difference between the expression levels of estrogen receptor alpha in males and females, but there was a significant increase in expression levels in response to 17 bet a -estradiol. Stimulation of benign and malignant thyroid cells with 17 bet a -estradiol resulted in an increased proliferation rate and an enhanced ex pression of cyclin D1 protein, which plays a key role in the regulation of G(1)/S transition in the cell cycle. In malignant tumor cells maximal cycli n D1 expression was observed after 3 h, whereas in benign cells the effect of 17 beta -estradiol was delayed. ICI 182780, a pure estrogen antagonist, prevented the effects of 17 beta -estradiol. In addition, 17 beta -estradio l was found to modulate activation of mitogen-activated protein (MAP) kinas e, whose activity is mainly regulated by growth factors in thyroid carcinom a cells. In response to 17 beta -estradiol, both MAP kinase isozymes, extra cellular signal-regulated protein kinases 1 and 2, were strongly phosphoryl ated in benign and malignant thyroid cells. Treatment of the cells with 17 beta -estradiol and MAP kinase kinase 1 inhibitor, PD 098059, prevented the accumulation of cyclin D1 and estrogen-mediated mitogenesis. Our data indi cate that 17 beta -estradiol is a potent mitogen for benign and malignant t hyroid tumor cells and that it exerts a growth-promoting effect not only by binding to nuclear estrogen receptors, but also by activation of the MAP k inase pathway.